Even so, the wall also served to keep out not just "casual migrants" but enemies, says Ian Haynes, an archaeology professor at Newcastle University. In the past decade, excavators have turned up extensive pits that had held posts, possibly for sharpened stakes, fronting parts of the eastern section of the wall. "The kind of effort that goes into these defenses isn't just for decorative purposes," says Haynes. "It's wise to think that they were doing this in deadly earnest." Archaeologists have long searched for traces of the tribes who lived north of the wall, partly to assess the threats the Romans faced.

After breakfast of beans and toast in the town of Twice Brewed, I again headed to the top of the Whin Sill, where the route goes up and down rocky crags. Cresting the trail's last big hill late in the afternoon, I saw the sunlit roofs of Carlisle, a town about ten miles to the west. Looking to the south across the (aptly named) Eden Valley was like paging through a picture book of 19th-century England. Cottages were tidily tucked among green-grid pastures threaded by wooded lanes. On the far side, a train chugged west.

A few miles on, I reached the village of Walton. After 18 miles of hiking, my only concern was getting off my feet. I unhooked a metal cattle gate and walked up a muddy path to Sandysike Farm. Built in 1760—probably with stones filched from the wall—the white farmhouse straddles the line of the wall, and the path runs along the back fence. Richard Sutcliffe, the owner, greeted me at the gate and led me into his messy, concrete-floored kitchen, where a three-legged black Lab, two Jack Russell terriers and four Jack Russell puppies competed for attention.

Over a mug of tea, Sutcliffe said that the new walking trail has been a blessing for the farms and towns along the wall's path. "It's harder and harder to make farming pay these days," he said. A few years ago, Sutcliffe and his wife, Margaret, converted an old stable into a bunkhouse. Between May and mid-September, the Sutcliffes are booked nearly solid; some of the hikers I met along the trail had made reservations nine months in advance. (To prevent erosion of the trail, authorities discourage visitors from walking it in the rainy season, from November to April.) Lured by the promise of Cumberland sausage made from local pork and a beer or two, I gingerly pulled my boots back on and headed up the road to the Centurion Inn, part of which stands atop the site of the wall.

In the six years since the Hadrian's Wall trail was designated a national landmark, more than 27,000 people have walked it from end to end. Some 265,000 hikers spend at least a day on the trail each year. Unesco has designated Hadrian's Wall and the ancient Roman border in Germany as part of a larger World Heritage site, the Frontiers of the Roman Empire; archaeologists and preservationists hope to add sites in other nations to outline the empire at its greatest.

Traveling the course of Hadrian's great fortification over six days, I got a sense of how the wall defined what it was to be Roman. Between Wallsend and Bowness-on-Solway, the western terminus, a line was drawn: Roman citizens and other cosmopolitan residents from across the empire on one side, barbarians (as the Romans termed everyone else) on the other.

On my last day, I crossed wide stretches of windy, flat fields and marshlands and munched on the last blackberries of the season as I headed to Bowness.

A white gazebo overlooking the Solway River marks the finish—or, for some, the start. A carved sign over the entrance reads "Wallsend 84 miles." A retired British sailor in an argyle sweater stood under the hut's roof. "We're at the end of the world out here," he said with a smile.

Berlin-based Andrew Curry last wrote for Smithsonian about Gobekli Tepe, a Neolithic temple in Turkey. Photographers Sisse Brimberg and Cotton Coulson live in Denmark.